With its large energy band gap and other unique physical properties, diamond is regarded as a desirable material for many engineering applications including wear-resistant tool coatings, optical windows for visible and infrared transmission, abrasives, and particularly high temperature electronic devices. Diamond can be used as a high-grade, radiation resistant, high-temperature semiconductor with potential application in many commercial, military, and aerospace technologies. Thus, there is considerable interest in finding and improving techniques for synthesizing diamond films for these and other applications.
The study of heteroepitaxial growth of diamond films is currently of great interest not only as a subject of basic research but also for a variety of applications of diamond films. By taking full advantage of its unique physical and chemical properties, such as high thermal conductivity and negative electron affinity, diamond may be an ideal material for the new generation of solid state electronic devices. (See, for example, G. S. Gildenblat, S. A. Grot, C. R. Wronski, and A. R. Badzian, Proc. IEEE 79, 647 (1991)).
Several new Schottky and ohmic contact structures, based on CVD diamond films, have been fabricated and evaluated during recent years. In the new emerging field of vacuum microelectronics, silicon remains as a favored cathode material because of its well-established microprocessing technology. It may also be desirable, from the applicable device point of view, to have a thin film of diamond on the silicon surface in order to enhance the operating current, as well as the stability of the device.
Various techniques for forming diamond films have been proposed. For example, U.S. Pat. No. 4,915,977 to Okamoto et al. proposes forming a diamond film by evaporating carbon onto the substrate by arc discharge at a carbon cathode and applying a negative bias voltage to the substrate so as to form a plasma glow discharge around the substrate. U.S. Pat. No. 4,830,702 to Singh et al. proposes a hollow cathode plasma assisted method and apparatus for forming diamond films. Unfortunately, such electrical discharge methods for forming diamond films often fail to produce high quality diamond films.
Microwave plasma enhanced CVD has also been used to form diamond films. In addition, techniques have been developed for enhancing the nucleation of diamond onto a silicon substrate, or other substrate, for the subsequent growth of a diamond film by a conventional growth process. For example, it is known that the diamond nucleation density on a substrate may be increased several orders of magnitude by simply scratching or abrading the substrate prior to placing it into the conventional CVD growth chamber. Although the size and density of grown diamond particles can be controlled to some extent by the size and density of the scratches, each diamond particle still grows in a random orientation. In addition, the maximum density of diamond nuclei is also typically limited to less than about 10.sup.9 /cm.sup.2.
Other attempts have been made to more effectively seed a nondiamond substrate, such as by spraying the substrate with diamond powder through an air brush, or by ultrasonically abrading the substrate surface. U.S. Pat. No. 4,925,701 to Jansen et al. proposes seeding a substrate with a diamond powder to enhance nucleation. Unfortunately, each of these types of preparation techniques has to be performed outside of the plasma CVD reaction chamber.
The scratching and seeding techniques, also fail to produce a surface which is sufficiently smooth to permit in-situ monitoring of the diamond growth rate. Therefore, ex-situ analysis is commonly used, such as cross-sectional scanning electron microscopy or profilometry. Such ex-situ analysis does not permit processing parameters to be controlled during the diamond growth process. Moreover, scratching or abrading a surface may destroy fine features, such as the tip of a field emitter.
An article entitled Generation of Diamond Nuclei by Electric Field in Plasma Chemical Vapor Deposition, by Yugo et al. appearing in Applied Physics Letters, 58 (10) pp. 1036-1038, Mar. 11, 1991, proposes a predeposition of diamond nuclei on a silicon mirror surface prior to the conventional diamond CVD growth process. A high methane fraction (i.e., at least 5 percent) in the feed gas is taught by Yugo along with a negative electrical bias of 70 volts negative with respect to ground applied to the substrate for a time period of just several minutes.
The Yugo article also proposes that a balance must be struck between the biasing voltage and the methane content of the gas mixture. The authors of the Yugo article theorize that an excessive acceleration of the ions from a high voltage can destroy newly formed diamond nuclei. Yugo suggests that revaporization of the newly formed diamond nuclei should be suppressed by mitigating the ion impact by keeping the magnitude of the bias voltage low. Thus, in order to offset the low bias voltage, the degree of carbon over saturation, as determined by the methane percentage, should be increased. Yugo reported that diamond nuclei growth did not occur below 5% methane content and that high densities of nuclei occurred only above 10% methane. In addition, the absolute value of the biasing voltages were maintained below 200 volts negative with respect to ground to avoid revaporization from high energy impacting ions. The total time duration for the pretreatment was limited to between 2 to 15 minutes.
Growth of epitaxial diamond thin films on Si (100), (110), and (111) flat substrates has been studied by several investigators. (See, for example, S. D. Wolter, B. R. Stoner, J. T. Glass, and et al., Appl. Phys. Lett., 62 (11), 1215 (1993); and X. Jiang, K. Schiffmann, A. Westtphal, and C. P. Klages, Appl. Phys. Lett., 63 (9), 1203 (1993)). Unfortunately, it is difficult to deposit a high density diamond film on non-planar surface, such as may be used in a field emitter. In addition, conventional field emitters, such as silicon field emitters, may suffer tip wear and oxidation, thereby reducing performance.
Previous studies have demonstrated that thin films of silicon carbide were producible on nanometer-scaled silicon field emitters without altering the geometrical structures of the emitters significantly as disclosed in J. Liu, U. T. Son, A. N. Stepanova, and et al., J. Vac. Sci. Technol. B12 (2), p. 717, Mar./Apr. (1994). Improved performance was achieved on these SiC coated emitters as compared to the pure Si ones.
Considering the facts of known diamond nucleation size and density, appropriate processing conditions must accommodate the highly curved surface geometries of the Si emitters. An article to Geis et al. entitled Diamond Cold Cathodes, from Applications of Diamond Films and Related Materials, p. 309-310 (1991) discloses a diamond mesa field emitter.
U.S. Pat. Nos. 5,258,685 and 5,129,850 each discloses covering a field emitter tip with a diamond coating by ion implantation of carbon atoms to provide a substantially uniform plurality of nucleation sites from which diamond growth is initiated. The patents further disclose that coating thickness on the order of 10 Angstroms to less than 5000 Angstroms are desirable and that irregularities in coating thickness and coverage be minimized. In addition, the implanted carbon nucleation sites discourages the formation of non-uniform coating which may include undesirably large crystallite growth.
U.S. Pat. No. 5,199,918 to Kumar discloses a diamond field emitter formed by selectively depositing a (111) oriented diamond film, forming a metal layer over the diamond film and etching the portion of the metal film to expose the peaks of the diamond crystals. U.S. Pat. No. 5,138,237 to Kane et al. and U.K. Published Application No. 2,260,641 to Miyata et al. disclose other diamond field emitters.
U.S. Pat. No. 5,141,460 to Jaskie et al. discloses an electron emitter including a layer of diamond on a selectively formed conductive or semiconductive electrode. Carbon ions are implanted at a surface of the electrode by an ion beam to thereby serve as nucleation sites for subsequent diamond deposition. Similarly, U.S. Pat. No. 5,290,610 to Kane et al. discloses a diamond coated silicon field emitter formed by disposing hydrocarbon and etchant reactant gasses together with the tip in a reaction vessel and providing an external voltage source such that electrons, emitted from the electron emitter, disassociate hydrocarbon constituents of the reactant gas. The constituents accelerate toward and are deposited onto the tip and are selectively etched by the etchant, such as hydrogen, so that the diamond form of the deposited carbon preferably remains. Unfortunately, a significant percentage of carbon may remain on the surface as nondiamond carbon, thus, reducing the effectiveness of the diamond. Accordingly, field emission characteristics and stability may be less than desirable.